This invention relates to an equalizer for use in equalizing an input signal of an intermediate frequency band in a digital radio transmission system.
In general, an equalizer of the type described is supplied through a transmission path with an input signal subjected to modulation at a modulation rate. The modulation may be, for example, quadrature amplitude modulation. Degradation of a characteristic in the transmission path and interruption in the transmission path give rise to a distortion of a waveform of the input signal. The equalizer serves to equalize the distortion of the waveform, as well known in the art.
A conventional transversal equalizer comprises a transversal filter having a central tap, a plurality of taps on both sides of the central tap, and a plurality of delay units which are connected between two adjacent ones of the taps and each of which provides a predetermined delay which is substantially equal to a reciprocal of the modulation rate. The input signal is derived from the central tap and the other taps as a central tap signal and additional tap signals, respectively. The central and the additional tap signals are multiplied by a central control signal and complex control signals supplied from a control signal circuit and are sent as gain controlled signals to a pair of adder circuits. Anyway, equalization of the input signal is carried out in the transversal filter and an equalized signal is produced as a filter output signal.
In order to produce the central and the complex control signals, zero forcing algorithm is often used in the transversal equalizer. With the zero forcing algorithm, each of the additional tap signals should fall within a predetermined range relative to the central tap signal on a vector plane. The predetermined range is, for example, between minus .pi./2 and plus .pi./2, both exclusive. Otherwise, the transversal equalizer can never carry out desired operation.
Phases of the other tap signals are often located outside of the predetermined range in dependency on a relationship between a carrier frequency of the input signal and the predetermined delay.
In another conventional transversal equalizer proposed in U.S. Pat. No. 4,453,256 issued to T. Ryu and assigned to the instant assignee, supplementary delay lines are added to a transversal filter to give supplementary delays to a preselected one of the additional tap signals and to a preselected one of the gain controlled signals. With this structure, it is possible to locate each phase of the preselected tap signal and the preselected gain controlled signal within the predetermined range.
The proposed equalizer, however, becomes intricate in structure because superfluous supplementary delay lines are added to the equalizer. In addition, inasmuch as each length of the supplementary delay lines should precisely be controlled or adjusted in order to locate each phase within the predetermined range. Such adjustment of each length requires much labor and time.